The present invention relates to intravascular stent implants for maintaining vascular patency in humans and animals and more particularly to a stent in the form of a braided stent.
Percutaneous transluminal coronary angioplasty (PTCA) is used to increase the lumen diameter of a coronary artery partially or totally obstructed by a build-up of cholesterol fats or atherosclerotic plaque. Typically a first guidewire of about 0.038 inches in diameter is steered through the vascular system to the site of therapy. A guiding catheter, for example, can then be advanced over the first guidewire to a point just proximal of the stenosis. The first guidewire is then removed. A balloon catheter on a smaller 0.014 inch diameter second guidewire is advanced within the guiding catheter to a point just proximal of the stenosis. The second guidewire is advanced into the stenosis, followed by the balloon on the distal end of the catheter. The balloon is inflated causing the site of the stenosis to widen. The dilatation of the occlusion, however, can form flaps, fissures and dissections which threaten reclosure of the dilated vessel or even perforations in the vessel wall. Implantation of a metal stent can provide support for such flaps and dissections and thereby prevent reclosure of the vessel or provide a patch repair for a perforated vessel wall until corrective surgery can be performed. It has also been shown that the use of intravascular stents can measurably decrease the incidence of restenosis after angioplasty thereby reducing the likelihood that a secondary angioplasty procedure or a surgical bypass operation will be necessary.
An implanted prosthesis such as a stent can preclude additional procedures and maintain vascular patency by mechanically supporting dilated vessels to prevent vessel reclosure. Stents can also be used to repair aneurysms, to support artificial vessels as liners of vessels or to repair dissections. Stents are suited to the treatment of any body lumen, including the vas deferens, ducts of the gallbladder, prostate gland, trachea, bronchus and liver. The body lumens range in diameter from small coronary vessels of 3 mm or less to 28 mm in the aortic vessel. The invention applies to acute and chronic closure or reclosure of body lumens.
A typical stent is a cylindrically shaped wire formed device intended to act as a permanent prosthesis. A typical stent ranges from 5 mm to 50 mm in length. A stent is deployed in a body lumen from a radially compressed configuration into a radially expanded configuration which allows it to contact and support a body lumen. The stent can be made to be radially self-expanding or expandable by the use of an expansion device. The self expanding stent is made from a resilient springy material while the device expandable stent is made from a material which is plastically deformable. A plastically deformable stent can be implanted during a single angioplasty procedure by using a catheter bearing a stent which has been secured to the catheter such as in U.S. Pat. No. 5,372,600 to Beyar et al. which is incorporated herein by reference in its entirety.
The stent must be reduced in size to facilitate its delivery to the intended implantation site. A coil stent is delivered by winding it into a smaller diameter and fixing it onto a delivery catheter. When the device is positioned at the desired site, the coil is released from the catheter and it either self-expands by its spring force or it is otherwise mechanically expanded to the specified dimension.
As with many stents, the deformation of the stent when it is assembled on the delivery catheter causes a strain in the stent material. If the strain is too large the material will experience plastic deformation to such an extent that the stent will not recover to the intended dimensions following deployment. This is true of superelastic or pseudoplastic alloys such as disclosed in U.S. Pat. No. 5,597,378 issued to Jervis, which is incorporated herein by reference in its entirety. Thus a maximum allowable strain based on material is a limiting parameter in stent design.
Two parameters influence the amount of strain a stent will experience during the deformation described above. The first is the degree of deformation applied to the stent and the second is the thickness of the stent material. For a given deformation, the strain experienced by a material is proportional to the thickness of the material. Since it is desirable to deliver a stent on the smallest delivery system possible it follows that the thickness of the stent material should be reduced to keep the strain within acceptable parameters. When forming a stent with a single solid strand (such a length of solid wire), a limit will be reached where the thickness of material becomes so small that the stent will meet the maximum allowable strain but will no longer have the hoop strength to provide adequate scaffolding.
Current helical coil stents are delivered on the smallest profile catheter that the stent will allow. Strain on the stent during assembly on the catheter is the limiting factor with stents made from solid round or flat wire helical coil stents.
U.S. Pat. No. 5,342,348 to Kaplan for xe2x80x9cMethod and Device for Treating and Enlarging Body Lumensxe2x80x9d discloses a single helically wound strand and two counterwound delivery matrix filaments. A two stranded stent is shown in U.S. Pat. No. 5,618,298 to Simon for xe2x80x9cVascular Prosthesis Made of Reasorbable Materialxe2x80x9d.
Mesh stents are disclosed in U.S. Pat. No. 5,061,275 to Wallsten et al. for xe2x80x9cSelf-Expanding Prosthesisxe2x80x9d, U.S. Pat. No. 5,064,435 to Porter for xe2x80x9cSelf-Expanding Prosthesis Having Stable Axial Lengthxe2x80x9d, U.S. Pat. No. 5,449,372 to Schmaltz et al. for xe2x80x9cTemporary Stent and Methods for Use and Manufacturexe2x80x9d, U.S. Pat. No. 5,591,222 to Susawa et al. for xe2x80x9cMethod of Manufacturing a Device to Dilate Ducts in Vivoxe2x80x9d, U.S. Pat. No. 5,645,559 to Hachtmann et al. for xe2x80x9cMultiple Layer Stentxe2x80x9d, U.S. Pat. No. 5,718,169 to Thompson for xe2x80x9cProcess for Manufacturing Three-Dimensional Braided Covered Stentxe2x80x9d.
Woven mesh stents typically have warp and weft members as disclosed in U.S. Pat. No. 4,517,687 to Liebig et al. for xe2x80x9cSynthetic Woven Double-Velour Graftxe2x80x9d, U.S. Pat. No. 4,530,113 to Matterson for xe2x80x9cVascular Grafts with Cross-Weave Patternsxe2x80x9d, U.S. Pat. No. 5,057,092 to Webster for xe2x80x9cBraided Catheter with Low Modulus Warpxe2x80x9d and EP 122,744 to Silvestrini for xe2x80x9cTriaxially-braided Fabric Prosthesisxe2x80x9d. The warp strands are typically the strands in the longitudinal direction on a prosthesis. The weft strands are typically the strands which are shuttled through warp strands to form a two dimensional array.
WO 95/29646 to Sandock for a xe2x80x9cMedical Prosthetic Stent and Method of Manufacturexe2x80x9d discloses a geometric pattern of cells defined by a series of elongate strands extending to regions of intersection and interlocking joints at regions of intersections formed by a portion of at least one strand being helically wrapped about a portion of another.
Various helical stents are known in the art. U.S. Pat. No. 4,649,922 to Wiktor for xe2x80x9cCatheter Arrangement Having A Variable Diameter Tip and Spring Prosthesisxe2x80x9d discloses a linearly expandable spring-like stent. U.S. Pat. No. 4,886,062 to Wiktor for xe2x80x9cIntravascular Radially Expandable Stent and Method of Implantxe2x80x9d discloses a two-dimensional zig-zag form, typically a sinusoidal form. U.S. Pat. No. 4,969,458 to Wiktor for xe2x80x9cIntracoronary Stent and Method of Simultaneous Angioplasty and Stent Implantxe2x80x9d discloses a stent wire coiled into a limited number of turns wound in one direction then reversed and wound in the opposite direction with the same number of turns, then reversed again and so on until a desired length is obtained.
Braiding is a well known craft. See Braidmaking by Barbara Pegg, published by A and C Black Ltd, 35 Bedford Row, London WC1R 4JH, pp. 9-16 which is hereby incorporated by reference.
It is an object of the invention to produce a stent which has the ability to tolerate greater deformations, yet has a smaller profile to permit the use of a smaller delivery system thereby reducing the amount of trauma experienced by the patient. It is a further object of the invention to produce a stent which would recover to specified dimensions with maximized radial hoop strength and resistance to lateral force.
The present invention is accomplished by providing an apparatus for a radially expandable stent for implantation within a body vessel, comprising one or more continuous, discrete, metal strands. At least three strands repeatedly cross over each other to form a bundle. The strands are joined at the proximal and distal end such that the strands are free to adjust their position relative to each other in response to compression forces. One or more bundles are wound together to form an elongate hollow tube.